Synthesis of High-Molecular-Weight Polypropylene Elastomer by Propylene Polymerization Using α-Diimine Nickel Catalysts

The α-diimine late transition metal catalyst represents a new strategy for the synthesis of atactic polypropylene elastomer. Taking into account the properties of the material, enhancing the molecular weight of polypropylene at an elevated temperature through modifying the catalyst structure, and further increasing the activity of α-diimine catalyst for propylene polymerization, are urgent problems to be solved. In this work, two α-diimine nickel(II) catalysts with multiple hydroxymethyl phenyl substituents were synthesized and used for propylene homopolymerization. The maximum catalytic activity was 5.40 × 105 gPP/molNi·h, and the activity was still maintained above 105 gPP/molNi·h at 50 °C. The large steric hindrance of catalysts inhibited the chain-walking and chain-transfer reactions, resulting in polypropylene with high molecular weights (407~1101 kg/mol) and low 1,3-enchainment content (3.57~16.96%) in toluene. The low tensile strength (0.3~1.0 MPa), high elongation at break (218~403%) and strain recovery properties (S.R. ~50%, 10 tension cycles) of the resulting polypropylenes, as well as the visible light transmittance of approximately 90%, indicate the characteristics of the transparent elastomer.


Introduction
As a very popular plastic material, polypropylene (PP) is used for many applications, and can be classified into isotactic polypropylene (iPP), syndiotactic polypropylene (sPP), and atactic polypropylene (aPP).Among these, unlike the low-molecular-weight aPP obtained as a byproduct of iPP [1], amorphous high-molecular-weight aPP is a kind of elastomer material with growing industrial application prospects.Its beneficial elasticity and optical, antioxidant, and anti-aging properties make it widely used as an adhesive, compatibilizer, and additive in a series of polymer materials [2][3][4].Only a few Ziegler catalysts [5], non-metallocene catalysts [6,7], and C s -, C 2v -and C 1 -symmetric metallocene catalysts [8][9][10][11][12][13] are available for the synthesis of atactic polypropylene.Notably, metallocene catalysts are preferred for the preparation of high-molecular-weight atactic polypropylene elastomers, but they are challenging to synthesize and sensitive to moisture and oxygen [14].
In the 1990s, the α-diimine nickel/palladium complexes [14,15] attracted the attention of researchers due to their easy synthesis, efficient catalytic activity, and good tolerance to moisture and oxygen.Due to the fast "chain-walking" property of α-diimine catalysts, they can catalyze ethylene to produce branched polyethylene [16][17][18][19][20][21].Formation of (1,ω) were synthesized and used in propylene homopolymerization to prepare polypropylene elastomers (Scheme 1).The influences of catalyst structure and polymerization conditions on catalytic activity, as well as the chain structure, molecular weight, and stress-strain properties of polypropylene products, were investigated.
Polymers 2024, 16, x FOR PEER REVIEW 3 of 15 α-diimine catalysts for ethylene polymerization, achieving good results in catalytic activity, thermal stability, and chain structure control of polyethylene [44][45][46].Based on these results, two α-diimine nickel catalysts with multiple hydroxymethyl phenyl substituents were synthesized and used in propylene homopolymerization to prepare polypropylene elastomers (Scheme 1).The influences of catalyst structure and polymerization conditions on catalytic activity, as well as the chain structure, molecular weight, and stress-strain properties of polypropylene products, were investigated.

General Methods and Materials
All experiments sensitive to moisture and air were in conformity with the specification for a dry and pure argon atmosphere using the standard Schlenk technique.Toluene and hexane were used after reflux distillation in an Ar atmosphere using sodium.Dichloromethane (CH2Cl2) was obtained by reflux distillation after dehydrating with CaH2.Diethylaluminum chloride (AlEt2Cl, 1.0 mol/L in n-hexane) was purchased from Yanfeng Technology Co., Ltd.(Shenyang, China).Methylaluminoxane (MAO, 1.0 mol/L in n-hexane) and triethylaluminum (AlEt3, 1.0 mol/L in n-hexane) were purchased from Zesheng Technology Co., Ltd.(Anqing, China).High-purity (99.9%) propylene gas was purchased from Dongrun Specialty Gases Co., Ltd.(Tianjin, China).The remaining chemicals were obtained through commercial means with no purification.

Characterizations
The FT-IR spectra of the ligands and corresponding nickel complexes were obtained by pressing KBr pellets using a Thermo Nicolet 6700 spectrometer (Thermo Fisher Scientific, Waltham, MA, USA).The 1 H NMR spectra of the ligands in CDCl3 at ambient temperature were acquired by Bruker DMX 400 MHz (Bruker Co., New Castle, DE, USA), with TMS as an internal standard; δ values are reported in ppm and J values in Hz.Elemental analysis was obtained using a Flash EA 1112 microanalyzer.The molecular weight Mw (weight average molar weight), Mn (number average molar weight), and distribution PDI (polymer dispersity index) of the polypropylene were determined in THF at 40 °C using the 1260 Infinity II system (Agilent Technologies, Inc., Santa Clara, CA, USA).The 1 H and 13 C NMR spectra of the structures of branched polypropylenes were obtained at 120 °C using deuterated 1,2-dichlorobenzene and TMS as the internal standard on the Bruker DMX 400 MHz system (Bruker Co., New Castle, DE, USA); δ values are reported in ppm.Differential scanning calorimetry (DSC) measurements were performed on a TA Instruments DSC Q20 instrument (PerkinElmer, Inc., Waltham, MA, USA).The glass transition temperatures (Tg) were recorded in the second heating run at 10 °C/min in an N2 atmosphere, with an instrument temperature range of −60 to 200 °C.Stressstrain experiments were performed using a CMT6104 instrument (NSS Laboratory Equipment Co., Ltd., Shenzhen, China) with specimens prepared on a HAAKE Mini Jet II system (Thermo Scientific, Inc., USA).UV-visible light transmittance was measured on a CARY 300 spectrophotometer (Varian Medical Systems, Inc., CA, USA) within the range of 400-800 nm.The catalytic activity equation is: A = m/(n × t), where "m" is the mass of

General Methods and Materials
All experiments sensitive to moisture and air were in conformity with the specification for a dry and pure argon atmosphere using the standard Schlenk technique.Toluene and hexane were used after reflux distillation in an Ar atmosphere using sodium.Dichloromethane (CH 2 Cl 2 ) was obtained by reflux distillation after dehydrating with CaH 2 .Diethylaluminum chloride (AlEt 2 Cl, 1.0 mol/L in n-hexane) was purchased from Yanfeng Technology Co., Ltd.(Shenyang, China).Methylaluminoxane (MAO, 1.0 mol/L in n-hexane) and triethylaluminum (AlEt 3 , 1.0 mol/L in n-hexane) were purchased from Zesheng Technology Co., Ltd.(Anqing, China).High-purity (99.9%) propylene gas was purchased from Dongrun Specialty Gases Co., Ltd.(Tianjin, China).The remaining chemicals were obtained through commercial means with no purification.

Characterizations
The FT-IR spectra of the ligands and corresponding nickel complexes were obtained by pressing KBr pellets using a Thermo Nicolet 6700 spectrometer (Thermo Fisher Scientific, Waltham, MA, USA).The 1 H NMR spectra of the ligands in CDCl 3 at ambient temperature were acquired by Bruker DMX 400 MHz (Bruker Co., New Castle, DE, USA), with TMS as an internal standard; δ values are reported in ppm and J values in Hz.Elemental analysis was obtained using a Flash EA 1112 microanalyzer.The molecular weight M w (weight average molar weight), M n (number average molar weight), and distribution PDI (polymer dispersity index) of the polypropylene were determined in THF at 40 • C using the 1260 Infinity II system (Agilent Technologies, Inc., Santa Clara, CA, USA).The 1 H and 13 C NMR spectra of the structures of branched polypropylenes were obtained at 120 • C using deuterated 1,2-dichlorobenzene and TMS as the internal standard on the Bruker DMX 400 MHz system (Bruker Co., New Castle, DE, USA); δ values are reported in ppm.Differential scanning calorimetry (DSC) measurements were performed on a TA Instruments DSC Q20 instrument (PerkinElmer, Inc., Waltham, MA, USA).The glass transition temperatures (T g ) were recorded in the second heating run at 10 • C/min in an N 2 atmosphere, with an instrument temperature range of −60 to 200 • C. Stress-strain experiments were performed using a CMT6104 instrument (NSS Laboratory Equipment Co., Ltd., Shenzhen, China) with specimens prepared on a HAAKE Mini Jet II system (Thermo Scientific, Inc., USA).UV-visible light transmittance was measured on a CARY 300 spectrophotometer (Varian Medical Systems, Inc., CA, USA) within the range of 400-800 nm.The catalytic activity equation is: A = m/(n × t), where "m" is the mass of polypropylene (g), "n" is the molar quantity of catalyst (mol), and "t" is the polymerization reaction time (h).

Propylene Polymerization
The polymerization experiments took place in a 100 mL stainless steel vessel furnished with a pressure control system and a temperature-controlled magnetic stirring apparatus.Initially, the reactor was evacuated at 100 • C for two hours to eliminate moisture, and then cooled to room temperature under an argon atmosphere before being purged twice with argon and once with propylene.Then, the solvent, cocatalyst, and catalyst were injected, and the polymerization experiment was carried out at a controlled temperature and pressure.After reaching the reaction time, the propylene supply was cut off, and the polymerization experiment was stopped.The resultant mixture was quenched by the addition of a 10 vol% HCl/C 2 H 5 OH solution and stirred for at least 6 h.The polypropylene was washed alternately with C 2 H 5 OH and H 2 O and dried in a vacuum oven at 60 • C for 8 h.

Synthesis of α-Diimine Ligands and Complexes C1 and C2
To study the effects of bulky substituents on the ligand's N-aryl group and acenaphthequinone-backbone on improving the catalytic activity and molecular weight of the polymer, we designed and synthesized two α-diimine nickel complexes, C1 and C2.
The synthetic procedures of α-diimine nickel(II) complexes are shown in Scheme 2. These ligands and complexes were characterized.The structure of α-diimine nickel complex C1 contained two hydroxymethyl phenyl substituents on the side of the acenaphthequinonebackbone, and there were three hydroxymethyl phenyl substituents on the para-site structure of N-aryl and acenaphthequinone-backbone of complex C2.The hydroxymethyl phenyl substituent plays an important role in improving catalytic behaviors.Using complexes C1 and C2 as catalysts for propylene polymerization, we observed that the resultant polypropylenes were transparent elastomers.The results are shown in Tables 1 and 2

Propylene Polymerization with α-Diimine Nickel (II) Complexes C1 and C2
Using complexes C1 and C2 as catalysts for propylene polymerization, we observed that the resultant polypropylenes were transparent elastomers.The results are shown in Tables 1 and 2. The results show that catalysts C1 and C2 had the maximum catalytic activity at 5.40 × 10 5 g PP/mol Ni•h and 4.53 × 10 5 g PP/mol Ni•h, respectively, and the activity still maintained above 10 5 g PP/mol Ni•h at 50 • C, indicating the good thermal stability of these catalysts.Compared with the α-diimine catalysts reported in the literature, these two catalysts showed good activity in catalytic propylene polymerization in toluene, a commonly used polymerization solvent in industry.This should be related to the hydroxymethyl phenyl substituents in the ligand structure.The hydroxymethylphenyl on the ligand is not a very greatly bulky group, but the hydroxy groups may react with the cocatalyst to form bulky substituents, and also react with the cocatalyst anion of the active center ion pair to increase the distance between cation species and anion, making propylene monomer insertion easier and improving thermal stability and catalytic activity [44][45][46].Meanwhile, the catalytic activity of catalyst C1 was higher than that of C2 under the same polymerization conditions, probably due to the absence of large steric hindrance substituents on the o-N-aryl of the C1 ligand, which was more conducive to the coordination and insertion of propylene.Furthermore, the effects of the cocatalyst type, Al/Ni molar ratio, polymerization temperature, and solvent on catalytic activity were investigated.The MAO, AlEt 2 Cl and AlEt 3 were used as cocatalysts for polymerization, respectively.The activity of the DEAC system was 3.67 × 10 5 g PP/mol Ni•h (Entry 2 in Table 1), while the catalytic activity of the MAO system was 0.93 × 10 5 g PP/mol Ni•h, and only trace polymer was obtained in the AlEt 3 system under the same polymerization conditions.This could result from the different activation abilities of the cocatalyst to the catalyst.The low-cost AlEt 2 Cl is a more efficient alkylaluminum cocatalyst because of its more suitable Lewis acidity and steric bulk for the α-diimine catalyst [47,48].C1/C2 activity was highest when the Al/Ni molar ratio was 1200.This is because too much cocatalyst may lead to an increase of chain-transfer reaction to the cocatalyst, resulting in a decrease in activity.
The experimental results show that polymerization temperature significantly affects activity.At lower polymerization temperatures, such as between −20 • C and 30 • C, the C1/C2 activity was significantly higher than that at higher polymerization temperatures.They exhibited the highest catalytic activity at 0 • C. When the polymerization temperature was increased above 50 • C, the decrease in activity was attributed to increased N-aryl rotations, leading to C-H activation between the metal center and the ortho-substituent of N-aryl and the formation of an inactive cyclized metal complex [49,50].In addition, the solubility of propylene gas in toluene decreases with an increase in temperature, resulting in a decrease in monomer concentration around the active center and thus in the rate of chain growth.The elevated temperature also accelerated the rate of chain transfer.
The pressure of propylene still has a certain effect on activity.The activity of C1 is 2.33 × 10 5 g PP/mol Ni•h at 0.1 MPa, and when the pressure was increased to 0.5 MPa, the activity increased to 3.67 × 10 5 g PP/mol Ni•h.However, the catalytic activity decreased to 2.87 × 10 5 g PP/mol Ni•h when the pressure further increased to 0.7 MPa.According to the literature [38], this phenomenon may be related to the fact that the higher monomer concentration is close to the active center of the catalyst, which could increase the temperature in the polymerization and decay the active centers quickly.We compared the polymerization of catalyst C1 in toluene and n-hexane, two commonly used industrial solvents.From the comparative data of Entry 2&9 (3.67 × 10 5 gPP/molNi•h & 3.10×10 5 g PP/mol Ni•h) and Entry 6&10 (1.93 × 10 5 g PP/mol Ni•h & 1.60 × 10 5 g PP/mol Ni•h), the activities of C1 in n-hexane solvent were slightly lower than in the toluene system, which was related to the lower polarity of n-hexane and the lower polarization degree of the ion pairs in the active center.The relative closeness of the cation metal center to the cocatalyst anion is not conducive to an insertion growth reaction of the monomer.Nevertheless, the catalyst exhibited good activity in n-hexane, which has not been reported in the literature.

Molecular Weight and Chain Structure of Polypropylene
Considering the material properties, especially the mechanical properties, it is an urgent problem to improve the molecular weight of polypropylene obtained by α-diimine.The literature states that the polypropylenes obtained from α-diimine nickel catalysts above room temperature (25 • C) had the highest M w of 717 kg/mol [51].At 30 • C, we obtained polypropylene with an M w of 925 kg/mol, offered by C2.Moreover, at 0 • C, polypropylene prepared by C1 had the highest molecular weight (M w = 1101 kg/mol), a value that is commonly 50-600 kg/mol in the literature [15,35,36,38,42,47].
It is widely accepted that the structure of the catalyst is an essential factor affecting the molecular weight of the polymer.The steric hindrance of substituent on the ligand has obvious effects on monomer insertion, chain growth, and the chain-transfer reaction.Therefore, the design of the catalyst structure should consider not only the increase in polymerization activity but also the impact on polymer molecular weight.In the above study of this work, the two catalysts C1 and C2 have shown good catalytic activity.We next investigated the molecular weights of polypropylenes prepared with these two catalysts.The GPC data show that the average molecular weights (M w ) of polypropylenes obtained from catalysts C1 and C2 at 30 • C were 736 kg/mol and 925 kg/mol, and the M w of the polypropylene was still above 400 kg/mol at 50 • C, which are higher than those of reported in literature.The molecular weight of the polypropylene was significantly improved using our provided catalysts with large steric hindrance substituent structures, and the high molecular weight of the polymer will further improve the mechanical properties of the material.In a comparison of the molecular weights of polypropylenes synthesized with these two catalysts, the molecular weights of polypropylenes prepared by catalyst C2 were higher than those prepared by catalyst C1 above 30 • C (Figure 1).This is due to the fact that the larger steric hindrance substituents of catalyst C2, both on acenaphthequinonebackbone and N-aryl, inhibit the chain-transfer reaction, resulting in longer polymer chains and higher molecular weights.Furthermore, there are some unique results.The M w of polypropylene obtained by catalyst C1 at 0 • C was as high as 1101 kg/mol, which is the highest molecular weight reported in the existing literature.Note also that Table 2 and Figure 1b indicate that polypropylenes with bimodal molecular weight distribution were prepared by catalyst C2 at the polymerization temperature of −20 • C and 0 • C.This could be explained by the asymmetric structure of C2.The difference between the two sides of the active center results in a significant difference in the monomer insertion reactions on either side of the active center, thereby forming different polymer molecular weights [40,52].However, with an elevated polymerization temperature, the selectivity of the monomer insertion direction decreased, and the GPC curves became a single peak.
obtained from catalysts C1 and C2 at 30 °C were 736 kg/mol and 925 kg/mol, and the Mw of the polypropylene was still above 400 kg/mol at 50 °C, which are higher than those of reported in literature.The molecular weight of the polypropylene was significantly improved using our provided catalysts with large steric hindrance substituent structures, and the high molecular weight of the polymer will further improve the mechanical properties of the material.
In a comparison of the molecular weights of polypropylenes synthesized with these two catalysts, the molecular weights of polypropylenes prepared by catalyst C2 were higher than those prepared by catalyst C1 above 30 °C (Figure 1).This is due to the fact that the larger steric hindrance substituents of catalyst C2, both on acenaphthequinone-backbone and N-aryl, inhibit the chain-transfer reaction, resulting in longer polymer chains and higher molecular weights.Furthermore, there are some unique results.The Mw of polypropylene obtained by catalyst C1 at 0 °C was as high as 1101 kg/mol, which is the highest molecular weight reported in the existing literature.Note also that Table 2 and Figure 1b indicate that polypropylenes with bimodal molecular weight distribution were prepared by catalyst C2 at the polymerization temperature of −20 °C and 0 °C.This could be explained by the asymmetric structure of C2.The difference between the two sides of the active center results in a significant difference in the monomer insertion reactions on either side of the active center, thereby forming different polymer molecular weights [40,52].However, with an elevated polymerization temperature, the selectivity of the monomer insertion direction decreased, and the GPC curves became a single peak.We further investigated the effect of the polymerization condition on the molecular weight of the polypropylene.When the polymerization temperature was −20 °C, the molecular weight of the polypropylene was not high due to the low chain growth.As the polymerization temperature rose, the rate of chain growth and monomer insertion accelerated, leading to an increase in the molecular weight of the polymer.However, as the polymerization temperature was further elevated to 50 °C, the molecular weight of the We further investigated the effect of the polymerization condition on the molecular weight of the polypropylene.When the polymerization temperature was −20 • C, the molecular weight of the polypropylene was not high due to the low chain growth.As the polymerization temperature rose, the rate of chain growth and monomer insertion accelerated, leading to an increase in the molecular weight of the polymer.However, as the polymerization temperature was further elevated to 50 • C, the molecular weight of the polypropylene decreased.This phenomenon is attributed to the increase in chain-transfer reactions and the corresponding decrease in the solubility of propylene monomers in toluene.The polypropylene had the highest molecular weight when the Al/Ni molar ratio was 1200.After a further increase in the Al/Ni molar ratio, the molecular weight of the polymer decreased because too many cocatalysts acted as chain transfer agents (Figure S7).When the polymerization pressure was increased from 0.5 MPa to 0.7 MPa, the M w of C1 increased from 736 kg/mol to 808 kg/mol (Figure S8).
It is evident from the literature that polypropylenes synthesized by α-diimine catalysts exhibit branches and are very similar to ethylene-propylene copolymers with adjacent methylene-sequence groups, which are associated with the "chain walking" and "chain straightening" that occur after β-H elimination.Thus, the microstructures of the polypropylene samples were characterized, and, according to [53], the branching density, [CH 3 ]/[CH 2 ] unit ratio, and 1,3-enchainment content can be presented by analyzing 1 H NMR (Figure S9).
As illustrated in Table 3, the resulting polypropylenes exhibited high branching densities (234-280/1000C), although these were lower than the theoretical value (333/1000C).This is attributed to the branching density being reduced by the presence of the methylene sequence generated from the 1,3-enchainment or "chain straightening" [32,33] with the values obtained at similar polymerization temperatures in the literature, the 1,3enchainment contents of the resultant polypropylenes prepared by C1 and C2 were still relatively low (3.57-16.96%).This indicates that the structure of the large steric hindrance α-diimine nickel catalyst can effectively inhibit the 1,3-insertion reaction that occurs during polymerization.The results show that ligand structure and polymerization temperature can effectively adjust the branching density and 1,3-enchainment content.The complex C2, featuring three large steric hindrance hydroxymethyl phenyl substituents, results in a significantly lower percentage of 1,3-insertion in the polypropylene structure compared to C1.This indicates that it is feasible to suppress the 2,1-and 1,3-insertion processes through modifications to the ligand structure in the α-diimine nickel system [34,40].Higher 1,3-enchainment contents were observed at higher polymerization temperatures, which may be caused by various factors, including the activation energy of chain walking and solution viscosity, etc. [34].
To further investigate the branches of polypropylene, we employed 13 C-NMR to characterize polypropylene samples (Figure 2) and analyzed these results in Table S1 and Figure S10 [54].Initially, it was observed that the chain structure of polypropylene has a clear methylene sequence generated from the 1,3-enchainment ([EEE], δ~29.84 ppm).In addition, alkyl side-chains resulting from "chain walking" have been observed.These include the presence of iso-butyl (iBu), 2-methyl hexyl (2MH), and long chains (L, more than 6 carbon atoms), confirmed by the presence at δ~23.0-24.0ppm, δ~22.68 ppm, and δ~13.97 ppm, respectively.However, some common branched chains of ethyl, propyl, and pentyl branches in polyethylene were not observed [42].The sequence structure was calculated, and the results are presented in Table 4.With the increase in temperature from −20 • C to 50 • C, the [EEE] content in the polypropylenes prepared by catalyst C1 increased from 6.4% to 13.3%.The change trend is consistent with the 1,3-enchainment content results of the 1 H-NMR calculation.As well as the structural changes in the main chain, the type and number of branches also increased with elevated temperature.Below 0 • C, the branches of iso-butyl, 2-methyl hexyl, and long chains can hardly be observed.However, the contents of [ELE], [EiBuE], and [E2MHE] increased to 0.8%, 1.3%, and 1.6% at 50 • C.This indicates that the elevated temperature promotes the occurrence of chain walking and leads to an increase in the type and number of branches on the polymer chain segments.The average sequence length of the E-unit was calculated from [53].The values of n E = 2.24, 2.32, 2.77, and 3.47 were concluded when elevating the temperature from −20 • C to 50 • C.Moreover, it was also observed that the polypropylene synthesized by catalyst C2, with its larger steric hindrance, had fewer branches than C1 at the same temperature.) [53].The values of nE = 2.24, 2.32, 2.77, and 3.47 were concluded when elevating the temperature from −20 °C to 50 °C.Moreover, it was also observed that the polypropylene synthesized by catalyst C2, with its larger steric hindrance, had fewer branches than C1 at the same temperature.The presence of branches and 1,3-enchainment on the polypropylenes further led to the irregularity of the chain structure, and the polymer chains could not be regularized for crystallization, as evidenced by the DSC curves (Figure 3).These polypropylenes did  The presence of branches and 1,3-enchainment on the polypropylenes further led to the irregularity of the chain structure, and the polymer chains could not be regularized for crystallization, as evidenced by the DSC curves (Figure 3).These polypropylenes did not exhibit melting temperatures T m within the range of −50 • C to 200 • C (Figure 3a).The glass transition temperature (T g ) was only observed between −36 • C and −23 • C, which was considerably lower than that of isotactic polypropylene (−10 to 5 • C).The T g of polypropylene prepared by catalyst C2 was consistently higher than that obtained from catalyst C1 under the same conditions.In Figure 3b,c, we can see that the T g of polypropylene increased when the polymerization temperature decreased from 50 • C to 0 • C, except for −20 • C. We believe that this change trend is closely related to the chain structure and molecular weight of the polymer.However, the molecular weight of polypropylene obtained at −20 • C is lower than that obtained at 0 • C. With the decrease in the molecular weight of the polymer, the proportion of the chain segments at the end of the chain increased, resulting in a decrease in the glass transition temperature.
pylene increased when the polymerization temperature decreased from 50 °C to 0 °C, except for −20 °C.We believe that this change trend is closely related to the chain structure and molecular weight of the polymer.However, the molecular weight of polypropylene obtained at −20 °C is lower than that obtained at 0 °C.With the decrease in the molecular weight of the polymer, the proportion of the chain segments at the end of the chain increased, resulting in a decrease in the glass transition temperature.

Properties of Polypropylene
The mechanical properties of polyolefin material prepared by late transition metal catalysts are very worthy of research.The amorphous polypropylene with higher molecular weight prepared by our catalysts exhibits improved elastic properties (Figure S11).Herein, the mechanical and elastic properties of the polypropylene samples were studied (Figure 4, Table S2).In the stress-strain curves of Figure 4a, the polypropylene samples demonstrated low tensile strength, ranging from 0.3 to 1.0 MPa, but high elongation at break from 218% to 403%.There is no obvious yield point on the stress-strain curves, showing the soft and tough characteristics of the elastomer [55].As the polymerization temperature increased from 0 °C to 50 °C, the tensile strength of polypropylene decreased from 1.0 MPa to 0.3 MPa, which is related to the decrease in the molecular weight of the polymer.Meanwhile, the elevated temperature resulted in an increase in the type and number of branches on the chain structure, leading to more entanglement between the polymer

Properties of Polypropylene
The mechanical properties of polyolefin material prepared by late transition metal catalysts are very worthy of research.The amorphous polypropylene with higher molecular weight prepared by our catalysts exhibits improved elastic properties (Figure S11).Herein, the mechanical and elastic properties of the polypropylene samples were studied (Figure 4, Table S2).
ture and molecular weight of the polymer.However, the molecular weight of polypropylene obtained at −20 °C is lower than that obtained at 0 °C.With the decrease in the molecular weight of the polymer, the proportion of the chain segments at the end of the chain increased, resulting in a decrease in the glass transition temperature.

Properties of Polypropylene
The mechanical properties of polyolefin material prepared by late transition metal catalysts are very worthy of research.The amorphous polypropylene with higher molecular weight prepared by our catalysts exhibits improved elastic properties (Figure S11).Herein, the mechanical and elastic properties of the polypropylene samples were studied (Figure 4, Table S2).In the stress-strain curves of Figure 4a, the polypropylene samples demonstrated low tensile strength, ranging from 0.3 to 1.0 MPa, but high elongation at break from 218% to 403%.There is no obvious yield point on the stress-strain curves, showing the soft and tough characteristics of the elastomer [55].As the polymerization temperature increased from 0 °C to 50 °C, the tensile strength of polypropylene decreased from 1.0 MPa to 0.3 MPa, which is related to the decrease in the molecular weight of the polymer.Meanwhile, the elevated temperature resulted in an increase in the type and number of branches on the chain structure, leading to more entanglement between the polymer In the stress-strain curves of Figure 4a, the polypropylene samples demonstrated low tensile strength, ranging from 0.3 to 1.0 MPa, but high elongation at break from 218% to 403%.There is no obvious yield point on the stress-strain curves, showing the soft and tough characteristics of the elastomer [55].As the polymerization temperature increased from 0 • C to 50 • C, the tensile strength of polypropylene decreased from 1.0 MPa to 0.3 MPa, which is related to the decrease in the molecular weight of the polymer.Meanwhile, the elevated temperature resulted in an increase in the type and number of branches on the chain structure, leading to more entanglement between the polymer chains and an increase in elongation at break.To further examine the polypropylene's capacity to revert to its original state upon tension release, a cyclic stress-strain test was performed to assess its elasticity.As shown in Figure 4b, the polypropylene sample was subjected to 10 repeated stress-strain cycles at 100% strain.After each cycle, it failed to revert completely to its initial state, with the first cycle resulting in the most significant deformation, suggesting both elastic and plastic deformation during the tension process.Even after 10 tension cycles, the strain recovery (SR) value of polypropylene remained at 50%, demonstrating its good elastic recovery capabilities.
Due to the irregular molecular chain structure, the resulting polypropylene cannot crystallize and exhibit transparency.The light transmittance of the polypropylene prepared by C1 was measured across the UV-visible light spectrum, ranging from 400 to 800 nm.As Figure 5 shows, polypropylenes at different polymerization temperatures demonstrated a high visible light transmittance of approximately 90%.
Due to the irregular molecular chain structure, the resulting polypropylene cannot crystallize and exhibit transparency.The light transmittance of the polypropylene prepared by C1 was measured across the UV-visible light spectrum, ranging from 400 to 800 nm.As Figure 5 shows, polypropylenes at different polymerization temperatures demonstrated a high visible light transmittance of approximately 90%.

Conclusions
In summary, two α-diimine nickel catalysts with multiple hydroxymethyl phenyl substituents were synthesized and used for propylene homopolymerization.Due to the presence of large steric hindrance substituents, the catalysts exhibited high activity and good thermal stability in toluene, enabling the preparation of high-molecular-weight homopolypropylene (up to 1101 kg/mol).Even at 50 °C, the catalytic activities were above 10 5 g PP/mol Ni•h, and the Mw of polypropylene reached 480 kg/mol.The polypropylenes prepared using asymmetric complex C2 exhibited bimodal molecular weight distribution at −20 and 0 °C, reflecting their unique catalytic properties.The resulting polypropylenes displayed methylene-sequence and branches in the chain structure, with high branching density (234~280/1000C), low 1,3-enchainment content (3.57~16.96%),and low Tg (−36~−23 °C).The low tensile strength and high elongation at break confirmed the elastomeric properties of the propylene.The structure of the catalyst and polymerization conditions significantly influence polymerization activity, enabling the regulation of polypropylene properties such as molecular weight, branching density, 1,3-insertion content, and stress-strain behavior.This suggests that modification of the catalyst structure is an effective means of controlling catalyst properties and polymer molecular weight.The exploration of improving the properties of elastomer further by adding comonomers, such as ethylene, and toughening isotactic polypropylene by in situ polymerization is in progress.

Table 1 .
The results of propylene polymerization catalyzed by complex C1 a .

Table 2 .
The results of propylene polymerization catalyzed by complex C2 a .